Properly implement neg_mechs & GM_USE_MG_CRED

SPNEGO was already using union creds.  Now make the mechglue know about
it, delete all of the cred-related SPNEGO stubs that are now not called
(lib/gssapi/spnego/cred_stubs.c), and implement gss_get/set_neg_mechs()
by storing the OID set in the union cred.

This commit was essentially authored as much if not more by Luke Howard
<lukeh at padl.com> as much as by the listed author.

lib/gssapi/mech/cred.h

@@ -37,6 +37,7 @@ HEIM_TAILQ_HEAD(_gss_mechanism_cred_list, _gss_mechanism_cred);
 
 struct _gss_cred {
 	struct _gss_mechanism_cred_list gc_mc;
+	gss_OID_set gc_neg_mechs;
 };
 
 struct _gss_cred *